"Suppose you're thinking about a plate of shrimp. Suddenly someone will say 'plate' or 'shrimp' or 'plate of shrimp,' out of the blue. No explanation and there's no point in looking for one either. It's all part of the cosmic unconsciousness."
Tracey Walter as "Miller" in "Repo Man"

I remember a practical joke some people played on another guy while I was in college. It involved putting a small amount of silver nitrate in a shower head (in case you aren't up on chemistry, silver nitrate is the stuff that in really tiny quantities turns black when it is exposed to light-- which is why it is used in photographic negatives).

That was a nasty, nasty joke. Although I can't say I felt all that much sympathy for the guy-- he had it coming.